Recently, there has been supplied a DC power supply system including a photovoltaic (solar) power generator or a household fuel cell.
However, an output power generated by solar cells is not stable because the amount of power generation changes due to weather conditions and temperature. Thus, it is generally provided with a secondary battery as a backup power source (see, e.g., Patent Document 1).
In the DC power supply system, a DC power supply such as a solar cell is connected in parallel with a circuit in which the secondary battery is connected in series with a parallel circuit of a DC/DC converter for charging the secondary battery and a DC/DC converter for discharging the secondary battery. Typically, the secondary battery has a voltage lower than a constant voltage of the DC power supply. When the voltage of the DC power supply is higher than the voltage of the secondary battery, a charging current flows in the secondary battery through the charging DC/DC converter to charge the secondary battery. In contrast, when the voltage of the DC power supply is lower than the voltage of the secondary battery, a discharging current flows from the secondary battery through the discharging DC/DC converter to supply an electric power from the secondary battery.
In the case of the photovoltaic power generator, the solar cell generates an electric power even when the electric power is not consumed at the load, and the electric power generated during this period of time is charged in the secondary battery. However, when the temperature of the solar cell itself becomes excessively high due to, e.g., an excessively sunny condition, the output voltage of the solar cell is rather reduced.
Patent Document 1 does not directly describe that the secondary battery is charged when the voltage of the DC power supply is lower than the voltage of the secondary battery, and only describes that the secondary battery can be charged by using, e.g., a DC/DC converter capable of stepping up a voltage. However, in the case of charging the secondary battery by stepping up the output voltage of the solar cell to be equal to or higher than the voltage of the secondary battery, the current flowing into the secondary side of the DC/DC converter becomes a value obtained by multiplying the current flowing into the primary side of the DC/DC converter by the conversion efficiency and the reciprocal of the boosting ratio, and the current value decreases considerably. In other words, the charging efficiency is reduced while the energy loss is increased.    Patent Document 1: Japanese Patent Application Publication No. 2008-48544